March 10, 2012

Austronesian spread across Indonesia

It is wonderful when different disciplines arrive at the same conclusions independently. Recent work in Austronesian languages by Gray, Drummond, and Greenhill arrived at the conclusion that Austronesian originated in Taiwan c. 5,200 years ago, and experienced a rapid expansion pulse c. 4,000 years ago that resulted in the colonization of island Southeast Asia.

This colonization event left its traces in the genomes of present-day eastern Indonesians; the pre-Austronesian inhabitants of those islands were Papuan-like, or, in terms of traditional physical anthropology Australoid, whereas the expanding Austronesian mariners were Mongoloid. If the expansion happened within the time frame in question, it can be dated by dense genotype data by looking at the relative lengths of Australoid and Mongoloid segments in extant individuals.

This is exactly what a new paper has achieved. It has determined that the "Asian" admixture signal in Eastern Indonesia dates to 4,000-3,000 years ago.

It shows an Austronesian origin in Taiwan around 5200 years ago, followed bya settlement pause (pause 1) between 5200 and 4000 years ago. After this pause, a rapid expansion pulse (pulse 1) led to thesettlement of Island Southeast Asia, New Guinea and Near Oceania in less than 1000 years.

which is entirely consistent with the 4,000-3,000 year old colonization of Eastern Indonesia deduced by genetic recombination analysis.

The paper has some brilliant visualizations of information, one of which is reproduced on the left. Notice the cline of Papuan ancestry in Eastern Indonesia, east of the Wallace Line.

From the paper:

These results strongly suggest that the admixture cline in East Indonesia reflects the spread of individuals of Asian ancestry coming from the west and admixing with resident groups of Papuan ancestry. Second, the time of admixture is highlyconsistent for the two different datasets, which were estimated bytwo different methods, and suggest that the admixture began about 4,000 y ago (Figs. 1E and 2E and Table 3). This time is in excellent agreement with estimates from archaeological and linguistic data for the arrival of Austronesian speakers in East Indonesia about 3,500–4,000 y ago (9, 21). For example, the oldest pottery found in East Indonesia, associated with the Austronesian culture, dates back to 3,500 y ago (22). Moreover , a Bayesian analysis of Austronesian languages dates the Austronesian expansion into Indonesia to about 4,000 y ago and also suggests a west to east spread across East Indonesia (9).

This is a wonderful co-validation of the (once controversial) use of Bayesian phylogenetics in linguistics, and time depth estimation via recombination in genetics, calibrated with "hard" archaeological evidence. It will be great to see the same consensus arrived for other families as well.

PNAS doi: 10.1073/pnas.1118892109

Genetic dating indicates that the Asian–Papuan admixture through Eastern Indonesia corresponds to the Austronesian expansion

Shuhua Xu et al.

Although the Austronesian expansion had a major impact on the languages of Island Southeast Asia, controversy still exists over the genetic impact of this expansion. The coexistence of both Asian and Papuan genetic ancestry in Eastern Indonesia provides a unique opportunity to address this issue. Here, we estimate recombination breakpoints in admixed genomes based on genome-wide SNP data and date the genetic admixture between populations of Asian vs. Papuan ancestry in Eastern Indonesia. Analyses of two genome-wide datasets indicate an eastward progression of the Asian admixture signal in Eastern Indonesia beginning about 4,000–3,000 y ago, which is in excellent agreement with inferences based on Austronesian languages. The average rate of spread of Asian genes in Eastern Indonesia was about 0.9 km/y. Our results indicate that the Austronesian expansion had a strong genetic as well as linguistic impact on Island Southeast Asia, and they significantly advance our understanding of the biological origins of human populations in the Asia–Pacific region.

4 comments:

Thanks for posting! The studies of autosomal genetic variations in this corner of the world are lagging behind, im my impatient opinion. As 1000 Genomes expand to SE and S Asia, the Pacific Islands remain in the shadows (and since parts of the region yielded strong migration flows into the US, the paucity of knowledge impacts medical applications of genetics).

But my real question is about what's described as Bayesian dating. Strictly speaking, Bayes applies only to homogeneous systems, and the heterogeneity of language development has been one of the main points of the critics of the old glottochronology. We've worked on post-Bayesian statistics in heterogeneous sets. To some degree the known sources of heterogeneity may be mitigated, but it isn't trivial, and one always needs to validate the results e.g. by using various partial subsets of the data. Do you know how the linguists approached this issue?

"Although the Austronesian expansion had a major impact on the languages of Island Southeast Asia, controversy still exists over the genetic impact of this expansion".

There hasn't been much controversy on the subject in this part of the world. Although a few make claims of pre-Polynesian 'Celtic' arrivals here. I have a book published in 1984 that is in complete agreement with all the processes and dates given in this paper. Of course it is nice to have yet more evidence in favour of what has been assumed for years.

"the genetic admixture between populations of Asian vs. Papuan ancestry in Eastern Indonesia. Analyses of two genome-wide datasets indicate an eastward progression of the Asian admixture signal in Eastern Indonesia beginning about 4,000–3,000 y ago"

I have had a few arguments in the past with those who maintain there was no southward Mongoloid movement involved. They claimed the Austronesian phenotype has always existed in SE Asia. I won't hold my breath waiting for any of them to admit they were wrong.

Does this mean there was only very small amount of mating going on between Austronesian and Papuans? 4000 years ago is an extremely long time. The population of East Indonesia was not even 500,000 by the time of 1800's. Which leads me to think that only around 10,000 Austronesians and Papuan mated in 2000BC while their descendants of millions of people population of East indonesia is attribute to the result of Genetic drift hybrid population by self intermixing. ( As we all know Hybrids inherit Y-DNA and mtDNA from their fathers and mothers)

I'm surprised this dated 4000 years ago. From what we know The East Indonesian have 70-80% Austronesian mtDNA with 10-20% Austronesian Y-DNA, 60-70% Papuan Y-DNA and 20-30% Papuan mtDNA. This obviously suggest more mating between Papuan men and Austronesian women. The question is how many exactly mated?

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